[Lldb-commits] [lldb] Don't count all the frames just to skip the current inlined ones. (PR #80918)

[via lldb-commits]

https://github.com/jimingham created https://github.com/llvm/llvm-project/pull/80918

The algorithm to find the DW_OP_entry_value requires you to find the nearest non-inlined frame.  It did that by counting the number of stack frames so that it could use that as a loop stopper.

That is unnecessary and inefficient.  Unnecessary because GetFrameAtIndex will return a null frame when you step past the oldest frame, so you already have the "got to the end" signal without counting all the stack frames.
And counting all the stack frames can be expensive.

>From c76edeec5c7430cd352c4d0ca977445800c55666 Mon Sep 17 00:00:00 2001
From: Jim Ingham <jingham at apple.com>
Date: Tue, 6 Feb 2024 17:30:17 -0800
Subject: [PATCH] Don't count all the frames just to skip the current inlined
 ones.

The algorithm to find the DW_OP_entry_value requires you to find
the nearest non-inlined frame.  It did that by counting the number
of stack frames so that it could use that as a loop stopper.

That is unnecessary and inefficient.  Unnecessary because
GetFrameAtIndex will return a null frame when you step past the
oldest frame, so you already have the "got to the end" signal
without counting all the stack frames.
And counting all the stack frames can be expensive.
---
 lldb/source/Expression/DWARFExpression.cpp | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/lldb/source/Expression/DWARFExpression.cpp b/lldb/source/Expression/DWARFExpression.cpp
index fe4928d4f43a43..c061fd1140fff7 100644
--- a/lldb/source/Expression/DWARFExpression.cpp
+++ b/lldb/source/Expression/DWARFExpression.cpp
@@ -608,11 +608,10 @@ static bool Evaluate_DW_OP_entry_value(std::vector<Value> &stack,
   StackFrameSP parent_frame = nullptr;
   addr_t return_pc = LLDB_INVALID_ADDRESS;
   uint32_t current_frame_idx = current_frame->GetFrameIndex();
-  uint32_t num_frames = thread->GetStackFrameCount();
-  for (uint32_t parent_frame_idx = current_frame_idx + 1;
-       parent_frame_idx < num_frames; ++parent_frame_idx) {
+
+  for (uint32_t parent_frame_idx = current_frame_idx + 1;;parent_frame_idx++) {
     parent_frame = thread->GetStackFrameAtIndex(parent_frame_idx);
-    // Require a valid sequence of frames.
+    // If this is null, we're at the end of the stack.
     if (!parent_frame)
       break;